1. Field of the Invention
This invention relates to variable frequency drives (VFDs) and more particularly to combining standard VFDs in a manner that reduces the harmonic distortion from levels that occur when standard VFDs are used separately.
2. Description of the Prior Art
VFDs provide variable speed control of motors by converting AC power to DC power and then converting the DC power to variable voltage and frequency AC power. In the conversion process harmonic disturbances are created in the incoming AC power feed. This distortion is proportional to the number of conversion channels or rectifiers (typically diodes) that are used in the VFD. The distortion frequencies are determined by the different harmonics of the AC supply frequency multiplied by:
(xn+/xe2x88x921)
where n represents the number of conversion rectifiers and x is taken from 1 to infinity.
In general, the magnitude of each of the harmonic disturbances falls off as n increases. Because of the lower magnitude of disturbances at higher harmonics, IEE519 primarily establishes standards for harmonic disturbances up to the 35th harmonic for six pulse drives which are the most common VFDs. The 35th harmonic then corresponds to a frequency level of 2100 Hz when the AC supply fundamental frequency is 60 Hz.
As was described above the most common VFDs are six pulse drives. Such drives have six rectifiers, two in each leg of a standard three-phase power line. To further reduce the magnitude of the harmonics and raise the frequency levels of the disturbances, 12-pulse and 18-pulse drives have been developed. These VFDs use phase shifting transformers to create multiple three phase power feeds which are phase shifted allowing the use of a greater number of rectifiers. The benefit of this arrangement is the cancellation of lower order harmonic disturbances. For example, 18-pulse drives would practically eliminate all disturbances below the 17th harmonic or 1020 Hz.
One use of VFDs is to simultaneously control multiple motors that are used in decanter type centrifuges. Decanter centrifuges are used in applications such as wastewater treatment to separate a fluid feed mixture into its constituent parts. A decanter centrifuge is typically comprised of a maindrive motor and a backdrive motor and thus has a maindrive VFD and a backdrive VFD. Some decanter backdrive motors act as regenerative devices that require a method for dissipating the regenerative energy produced by the centrifuge process.
As is described in U.S. Pat. No. 5,203,762 the dissipation of the regenerative energy can be achieved by connecting the backdrive VFD to the maindrive VFD through a common DC buss or with a separate DC to AC converter to put energy back unto the AC power line. Although this patent describes a method for recouping the energy of the backdrive motor, it does not address the problem of harmonics disturbances created by the 6-pulse front end. The invention described herein uses features of the common buss VFD and features of the 18-pulse drive system to create a clean power common buss drive system for decanter centrifuges.
The present invention is a N pulse variable frequency drive for a decanter centrifuge. The centrifuge has a bowl rotatable about its longitudinal axis and a conveyor, a main drive motor for rotating the bowl and a back drive motor for rotating the conveyor. The drive has:
(a) a M1 pulse variable frequency drive connected to the main drive motor;
(b) a M2 pulse variable frequency drive connected to back drive motor;
where M1+M2=N;
(c) a common DC buss connected to the M1 and M2 pulse variable frequency drives; and
(d) a N phase shifting transformer connected to the M1 and M2 pulse variable frequency drives.
The present invention is also a N pulse variable frequency drive for driving two or more motors that has:
(a) a M1 pulse variable frequency drive connected to drive the first of the two or more motors;
(b) a M2 pulse variable frequency drive connected to drive the second of the two or more motors;
(c) a common DC buss connected to the M1 and M2 pulse variable frequency drives; and
(d) a N phase shifting transformer connected to the M1 and M2 pulse variable frequency drives.
The present invention is also the combination of a decanter centrifuge and a N pulse variable frequency drive. The decanter centrifuge has a bowl rotatable about its longitudinal axis; a conveyor; a main drive motor for rotating the bowl; and a back drive motor for rotating the conveyor. The drive has a M1 pulse variable frequency drive connected to the main drive motor; a M2 pulse variable frequency drive connected to the back drive motor; where M1+M2 N; a common DC buss connected to the M1 and M2 pulse variable frequency drives; a N phase shifting transformer connected to the M1 and M2 pulse variable frequency drives.
The present is further a method for driving a decanter centrifuge using a N pulse variable frequency drive. The centrifuge has a bowl rotatable about its longitudinal axis, a main drive motor for rotating the bowl and a back drive motor for rotating the conveyor, and a conveyor. The method has the steps of:
(a) connecting a M1 pulse variable frequency drive to the main drive motor;
(b) connecting a M2 pulse variable frequency drive to the back drive motor;
where M1+M2=N;
(c) connecting a common buss to the M1 and M2 pulse variable frequency drives; and
(d) connecting a N phase shifting transformer to the M1 and M2 pulse variable frequency drives.